Controlled flux addition for minimizing surface defects on continuously cast steel

ABSTRACT

In the continuous casting of steel, a low melting point flux is applied to the top surface of the molten steel in the mold, to provide adequate assimilation of scum and thus minimize breakouts. In using such low melting point fluxes, it has been found that pinhole defects can be substantially eliminated by feeding an amount significantly lower than is conventionally employed. The maximum amount which may be employed is dependent on the dimensions of the caster and is given by the equation:

United States Patent 1 Szekeres June 24, 1975 [54] CONTROLLED FLUX ADDITION FOR 3,318,322 Solis 3,642,0 c rewe et a TS ON 3,649,249 3/1972 l-lalley et a1 164/82 3,704,744 12/1972 Halley et a1 164/73 Inventor:

Assignee:

Filed:

Appl. No.:

US. Cl. 164/55; 164/82; 164/73; 164/52 Int. Cl B22d 27/18 Field of Search 164/73, 82 (US. only), 164/48, 50, 52, 55 (US. only) Edward S. Szekeres, Edgewood Boro, Allegheny County, Pa.

United States Steel Corporation, Pittsburgh, Pa.

Oct. 31, 1972 References Cited UNITED STATES PATENTS PEflCE/VT 0F CAST SURFACE CONTAIN/N6 P/NHOLES Goss 164/73 Hamilton 164/73 Primary ExaminerFrancis S. l-lusar Assistant Examiner-V. K. Rising Attorney, Agent, or FirmArthur J. Greif l 5 7] ABSTRACT F max. 4 Claims, 1 Drawing Figure FLUX-USAGE COEFF/C/ET PATENTED JUN 24 m5 RMGFQMQQ m6 $5 is i CONTROLLED FLUX ADDITION FOR MINIMIZING SURFACE DEFECTS ON CONTINUOUSLY CAST STEEL This invention relates to a method for continuous casting of steels and is specifically directed to the controlled addition of synthetic fluxes.

In the continuous casting of aluminum deoxidized steels, special precautions are usually taken to prevent the formation of excessive amounts of alumina scum which forms when A1 0,; reacts with iron. silicon and manganese oxides. Several methods have been utilized commercially in attempts to either prevent the oxidation of aluminum and/or to alter the chemical nature of the oxidation products With respect to the latter expedient, it is known (see. for example. US. Pat. No. 2.825.947) that the addition of flux materials to the top surface (meniscus) of the molten steel can greatly improve casting quality. The fluxprevents the accumulation of oxide scum on the meniscus of the liquid steel. Fluxes are often added, for a variety of reasons, even in cases where the steel does not contain appreciable amounts of aluminum. For example, the flux can (a) act as an insulating layer to prevent heat loss from the molten metal. (b) provide a barrier to the penetration of oxygen into the liquid steel in the mold. and provide a desired level of lubrication at the shell-mold interface. Although a large variety of fluxes have been employed. it has been found that superior results are achieved by employing low melting point fluxes, i.e., those which melt very quickly and essentially flow down the sides of the shell of the strand, as fast as they are added to the mold. Examples of these fluxes are shown in US. Pat. No. 3,649,249. It has now been found that when utilizing such low melting point fluxes; it is particularly desirable to employ an amount of flux within very specific limits in order to minimize pinhole defects. Pinhole defects are small holes up to about 0.13 inch in diameter and up to about 0.15 inch deep that appear on the surface of the cast strand. When present on the surface or sufficiently close to the surface. the pinhole becomes oxidized and results in seams and slivers on the product rolled from the cast material.

It is therefore a principal object of this invention to provide a method for the continuous casting of steel strands with a minimum of surface defects.

This and other objects and advantages of the instant invention will be more apparent from the following description and claims when read in conjunction with the drawing, in which The FIGURE is a plot of pinhole frequency vs. flux usage coefficient.

As stated above. the fluxes for which the instant invention is applicable are those which exhibit a comparatively low melting temperature. Particularly desirable fluxes are those which have a final chemical composition within the following range, in wt. percent:

('aF- to 60% The above fluxes may be produced by blending together a source of calcium fluoride. alkali oxides (sodium or potassium). sodium borates and lime and melting the mixture to produce a lowsoftening point final product which is completely homogeneous and from which substantially all the gases have been removed. The molten material is then quenched, crushed and sized to produce the final product used in the casting procedure. In any case, whether the above preferred fluxes are employed, or others well known to the art (e.g. US. Pat. No. 3,649,249) it is essentialthat they exhibit a melting point within the range of l400 to I900F and preferably within the range 'of I500" to 1750F. Since the product is essentially vitreous, there is no clearly defined melting point. However, for purposes of this invention the melting point may easily be determined by the following or other equivalent method:

Aone-inch high by three-quarteninch diameter, cylindrical pellet is pressed from the prefused, crushed and dried flux. The pellet is placed on a refractory slab (about 2 X 2 X 1 inch) and inserted in a preheated furnace at l400F. After 15 minutes, the pellet is observed for the extent of melting. This test is then repeated (starting with a fresh pellet) every 25F until complete melting is observed in the respective 15-minute hold period. Complete melting is that condition for which none of the initial pellet profile is observed, i.e., the flux is sufficiently liquid to flow and substantially cover the slab surface. The temperature at which such complete melting" is observed may then be considered the melting point of the synthetic flux.

Experience with a number of different low-melting point flux compositions has shown that it is preferable to add sufficient flux to provide a film, between the strand shell and the mold, which is at least about 0.0003 inches thick. It was found that the amount of flux to provide a film this thick was adequate to achieve desired asimilation of scum and thus ensure against frequency breakouts. It was generally reasoned that the more flux added, the less the chance of scum. However, it has been discovered that in the continuous casting of aluminum-containing steels, there is a very definite maximum amount of flux, above which the pinhole frequency of the surface markedly increases. Ironically, aluminum (greater than 0.010%) is often added to many strand-cast steels to prevent such pinhole defects. Similar effects of excess flux addition were also noted for steels in which no aluminum is added, e.g. silicon deoxidized steels. Experimental work leading to the instant invention was conducted on a 7.5 inch-square billet mold. To relate these findings to molds of various dimensions (i.e., both billet and slab casters) the abscissa of the FIGURE is drawn to a flux-usage coefficient, whereby the requisite usage may be determined for virtually any conventional size mold (i.e., those with top surface areas ranging from about 0.1 ft for billets to 6 ft for slabs). It may readily be seen, that when the flux usage is in excess of about 0.017, there is a marked increase in pinhole frequency. For such conventional molds. the maximum-usage (F in pounds of flux per ton of steel cast), is given by the equation:

where C.S.A. is the calculated surface area of a one-ton steel billet which may be covered by an essentially liquid flux. Similarly, the preferred minimum flux usage (F is given by the equation:

F 0.0022 (C.S.A.)

The following two examples illustrate how the prescribed coefficients may be employed to determine the amount of flux (in lbs per ton of steel cast) to be added. 0.5 foot X 2 foot casting mold One ton of molten steel occupies a volume of approximately 4.6 cu. ft. Therefore, the length of a one-ton steel slab within the prescribed mold will be 0.5 X 2 X L 4.6, or L 4.6 ft. The covered surface area (C.S.A.) of such a slab will then comprise the top surface (0.5 X 2) and the four sides 2 (0.5 X 4.6) and 2 (2 X 4.6), or 24.0 ft. Thus the maximum flux usage, in lbs. per ton of steel, for this mold would be:

F mum or, approximately 0.42 lbs/ton. Similarly, the minimum should preferably be:

or, about 0.053 lbs/ton. 0.625 foot X 0.625 foot casting mold The length of a one-ton steel billet for this mold is 4.6/.39 l 1.8 ft. The covered surface area is therefore approximately 29.9 ft'. Thus,

F mum i.e., about 0.51 lbs/ton and maximum of this invention.

1 claim:

1. In the method of continuously casting of steel. which includes the addition of a readily fusible flux onto the meniscus of the molten steel, in an amount sufficient to aid in the assimilation of scum, said flux comprising a prefused mixture of, in weight percent,

C HF: 20 to 60% C210 5 to 30% K O/Na O 10 to 30% B 0 5 to 20% SiO 15% max. A -gQl 5% max. C 10% max.

and exhibiting a melting point within the range of about l400 to 1900F,

the improvement which comprises, feeding said flux at a rate below that given by the equation:

in which, F,,,,,,,. is pounds of flux per ton of steel, and C.S.A. is the calculated surface area, in square feet, of a one ton steel billet being cast and being covered by an essentially liquid flux. 2. The method of claim 1, in which the minimum flux feed rate is at least that given by the equation:

F 0.0022 (C.S.A.).

3. The method of claim 2, in which said steel has an aluminum content in excess of about 0.010 percent.

4. The method of claim 2, in which said flux exhibits a melting point within the range of about I500 to l750F.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION 5,891,023 June 2A, 1975 Patent No. Dated hwentofls) Edward S. Szekeres It is certified that error appears in the ahove-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2, line 1, "lowso'ftening" should read low-softening lines 33-39, "frequency" should be vzweqpent Column 3, line 6, "0.5 foot X 2 foot casting mold should be 0.5 foot x 2 foot casting mold "01",- about 0.053 lbs/tori. 0.625 foot X' 0.625 zoot casting" mold The length of a one-ton steel billet for this"- should beor, about 0.053 lbs/ton.

0.625 f'oot 'X 0.625 foot casting mold The length or? a @e-to? teel billet For this Signed and Scaled this seventh Day of October 1975 SEAL] A ttest:

RUTH C. MASON C. MARSHALL DANN' Commissioner of Patents and Trademarks A nesting Officer 

1. IN THE METHOD OF CONTINUOUSLY CASTING OF STEEL, WHICH INCLUDES THE ADDITION OF A READILY FUSIBLE FLUX ONTO THE MENISCUS OF THE MOLTEN STEEL, IN AN AMOUNT SUFFICIENT TO AID IN THE ASSIMILATION OF SCUM, SAID FLUX COMPRISING A PREFUSED MIXTURE OF, IN WEIGHT PERCENT, CAF2 20 TO 60% CAO, 5 TO 30% K2O/NA2O 10 TO 30% B2O3 5 TO 20% SIO2 15% MAX. AL2O3 3% MAX. C 10% MAX. AND EXHIBITING A MELTING POINT WITHIN THE RANGE OF ABOUT 1400* TO 1900*F, THE IMPROVEMENT WHICH COMPRISES, FEEDING SAID FLUX AT A RATE BELOW THAT GIVEN BY THE EQUATION:
 2. The method of claim 1, in which the minimum flux feed rate is at least that given by the equation: F min 0.0022 (C.S.A.).
 3. The method of claim 2, in which said steel has an aluminum content in excess of about 0.010 percent.
 4. The method of claim 2, in which said flux exhibits a melting point within the range of about 1500* to 1750*F. 